Gypsum is calcium-sulphate dihydrate (DH) of the formula CaSO4.2H2O. Vast deposits of natural gypsum provide gypsum rock or gypsum sand. Synthetic gypsum originates from the phosphoric acid production and more and more from Flue Gas Desulphurization (FGD).
Plaster, in this context and in the generally accepted terminology of the art, is partially dehydrated gypsum of the formula CaSO4.xH2O where x=0 to 0.5, with the potential to re-crystallize to a solid structure when mixed with an appropriate amount of water.
Calcining means the thermal treatment of a DH in order to remove a part of the combined water.
Hemihydrate (HH) or semihydrate (SH) is the metastable hydrate of the formula. CaSO4.½H2O.
Anhydrite III (AIII) is a dehydrated HH with the potential of reversibly absorbing water or even vapor. The reversible uptake of water liberates considerable reaction heat.
Anhydrite II (AII) is the completely dehydrated product. It is formed at higher temperatures and is not welcome in stucco plasters and therefore, in calcining for industrial plasters, conditions to create AII are avoided as far as possible.
HH and AIII are the products resulting from the first steps of calcining. Whether AIII or HH is first formed depends on the calcining temperature and the vapor pressure in the calcining ambience.
Generally plasters are calcined under dry conditions meaning in hot air or in an indirectly heated calcining vessel. Under those conditions the size and shape of the DH particle of origin remains essentially the same. Thus the resulting plaster is porous. It is ordinarily called stucco or plaster of Paris. The accepted technical term is β-Hemihydrate (β-HH).
The possible use of plasters as a binder results from its ability to build up a completely new crystalline structure out of an aqueous slurry. This is due, in the first place, through the very big difference in the solubilities of HH and DH (about 8 g/l vs. 2.7 g/l). Thus, a HH creates a tremendous over-saturation with regard to DH. The over-saturation leads to the formation of germs and quick re-crystallization sets in.
Normally, salts increase their solubility with the temperature. Ca-Sulphate behaves rather irregular in decreasing its solubility. The solubility curves of HH and DH cross each other at about 100° C. In the temperature range between 85 and 100° C. the differences in solubility are so small that setting virtually does not start at all. At 75° C. the reaction rate is still very low.
Due to the rough thermal treatment the physical microstructure of β-HH is stressed and quite unstable. Thus, one observes that, in contact with liquid water, a β-HH will partially disintegrate into very small particles. However, by absorbing humidity, the stress is lowered and the disintegration phenomenon fades. Simultaneously the speed of dissolution in water diminishes. The phenomenon is called “ageing”. The term is quite misleading because it is more an effect of the ambient conditions (humidity, temperature) than of time.
Because of ageing, β-HH has the tendency to change its rheology and setting kinetics over the time dramatically. The drift in rheology is caused by the diminishing tendency of β-HH to disintegrate, as explained above, in very fine particles. The drift in kinetics has to do with the “healing” of crystalline defects (spots of heightened activity) in the calcined product.
The starting point of the drifting properties depends largely upon the origin of the plaster, the granulometry and the calcining conditions. There is a widely accepted consensus that the adsorption of water is the main promoter of ageing. AIII can take up as much humidity as to become HH. Then, surprisingly, the uptake of water continues until about 8% combined water, which is significantly above the theoretical value of HH, without forming DH.
Ageing is a problem in construction/wall plasters where the conditions of storage and the delay between calcining and application can vary in a wide range. In plasterboard production ageing is a problem as well, albeit to a lesser extent.
Plasters, which have reached their virtual final state of ageing, offer two main advantages:
a) constancy and reliability;
b) control of the granulometry and, thus, of the rheology.
This has the effect of for example, without being limited thereto:                less overall variation in product qualities;        less water to dry out in plasterboard manufacture; and        less retarder in construction/wall plasters;        less ultrafines in gypsum fiber boards, resulting in an easier dewatering.        
The art knows since long time how to age a plaster forcedly. The basic idea is quite simple: give all, or even more, of the water at once that is needed to quench the “thirst” of the plaster. The process has been called “stabilization” in the prior art.
Note that forced ageing or stabilization in the sense it is used in the present invention is not “aridisation” which is essentially calcining in the presence of deliquescent substances (see e.g. U.S. Pat. No. 1,370,581).
U.S. Pat. No. 1,713,879 is apparently the first publication dealing with stabilizing. It discloses the mixing with water and/or steam with a calcined plaster. The purpose is to reduce the water demand and the temperature rise during setting. The figures are: 12 to 15 pounds of water/minute for one ton of plaster over a period of 5 to 6 min (equivalent to 5 to 9% water in total). The plaster is preferably a “single boil plaster” (i.e. essentially HH without AIII). It is a batch type operation. There is no mention of temperatures or specific features of the equipment used. A variant is the introduction of water by means of a carrier like diatomaceous earth. The process is called (forced) ageing and not yet stabilizing.
DE-A-553519 discloses a process of treating calcined plaster with water and/or steam in order to render the plaster less sticky and less water demanding. The amount of water absorbed is 0.5 to 7%. It uses the reaction heat of AIII in order to heighten the temperature. The temperature at the end is between 80 and 130° C. and should not exceed 140° C. The patent does not disclose limits for the curing time but gives an example of half an hour of treatment of plaster with the exhaust gases of a rotary kiln. The temperature of the plaster discharged is 95° C. There is no mention about drying but. There is a mention that the treatment can be done in rotary apparatus, which allows an intimate contact of the steam with the product.
U.S. Pat. No. 1,999,158 mentions and relies apparently upon U.S. Pat. No. 1,370,581 (aridisation). The field of application is wall plasters. The claimed improvement lies in the superfine-grinding in order to increase the plasticity and to reduce the change in setting time over elapsed storage time of the powder. Plasticity is defined by the US consistency of 65 to 75. The fineness of the ground plaster is described as having a large part smaller than 10 μm. (Note that the term stabilized plaster is first used).
U.S. Pat. No. 2,177,668 deals with forced ageing which is in this case essentially the reversion of AIII to HH by the treatment of the calcined plaster with huge amounts of air of about ambient RH (60% RH) and a temperature just below the theoretical stability temperature of DH at 42° C.
U.S. Pat. No. 3,415,910 discloses quenching of a hot plaster with water whilst maintaining a temperature high enough to avoid the formation of DH (between 82 and 100° C.) and subsequent heating above 102° C. (drying up to 157° C.). The moisture content at the highest was 3%. The drying was done up to the point where the theoretical value of combined water for HH was attained. The preferred (and exclusively described method) was using a kettle as calciner and utilizing the same kettle as the device for the treatment and subsequent drying step. The plaster obtained and claimed is characterized by: (i) density at 20° C.=2.60 g/ml (<10% below 1.6 and <10% above 2.68 g/ml) and (ii) stacking order index above 8. The patent describes the role of disintegration on water demand and the rheological properties.
GB-A-1233436 is essentially equivalent to U.S. Pat. No. 3,415,910. However some slight differences and additional information are disclosed, suggesting that the process has been further developed. For example the maximum moisture has risen to 3.5%, the admissible calcining temperature is now 160° C. Treatment temperature in laboratory could be as low as room temperature. A preferred treatment temperature in industrial application is between 82 and 93° C. A preferred drying temperature is above 115° C. Graphs demonstrate the effect of free moisture and of curing time on the US consistency suggesting that 3% at 3 min are the lower limits of operation.
U.S. Pat. No. 3,527,447 is an improvement over U.S. Pat. No. 3,415,910. It discloses the drying step carried out in a separate device under sub-atmospheric pressure. In order to maintain the required temperature range an additional energy input by means of microwaves is suggested.
U.S. Pat. No. 4,117,070 (and related U.S. Pat. No. 4,153,373 and FR-A-2383893) proceeds to a continuous method for stabilizing without drying as part of a plasterboard production process. In a specific embodiment 50 to 75% of the board line feed are treated with 1 to 8% of free water cured for about one minute and this feed is then recombined and mixed with the remaining portion of the feed which is cured another three minutes. The total moisture after recombination is 3-4%. Is disclosed a fluidized and agitated vessel as a wetting vessel.
EP-A-0008947 deals with the inconvenience of storing a longer time wetted plaster. It introduces the notion of “set suddenness” which is the maximum temperature rise during setting. High set suddenness is disclosed as essential for the development of an adequate mechanical resistance and is substantially reduced by the stabilization procedure. The remedy for this drawback is grinding the treated (dried or not) plaster to a fineness 3 to 4 times the original (measured in Blaine).
GB-A-2053178 discloses the simultaneous grinding and wetting in an “Entoleter” mill or the like. Curing happens after the size reduction. The set suddenness of EP-A-0008947 is attained by this procedure as well.
Those patents differentiate between forced ageing (i.e. quenching/moistening) and stabilizing (i.e. quenching/moistening and curing, and optionally drying). They claim different approaches to obtain an aged or a stabilized plaster and specify appropriate granulometries for the use in plasterboard production.
Every stabilizing method includes the steps of moistening and curing. Moistening is the trickiest part, but curing has some problems as well. Two main concerns are: (1) unintended rehydration, which creates DH, acting as crystallization seeds in plaster slurries and (2) built-ups or scaling in the equipment.
The formation of DH occurs if liquid water and plaster are in contact over a certain time under thermodynamic conditions allowing the reaction that is at lower temperatures. It is obvious that the moistening of a binder like plaster leads inevitably to the formation of lumps and that every surface in contact with the moistened product and/or the moistening liquid has a tendency to build up crusts of potentially hardened matter. The problems are more pronounced in the moistening part of the devices because moistening includes mixing which produces dust and includes the presence of water which can lead to condensation.
What has been disclosed in the related art with regard of solving the problems mentioned above is not satisfying. In U.S. Pat. No. 3,415,910, the use of a kettle with the need to cool down and reheat the whole equipment is time and energy costly. U.S. Pat. No. 4,153,373 describes as a wetting apparatus a fluidized and agitated vessel used in the process of treating plaster for plasterboard production. Here the formation of traces of DH does no harm because the plaster is to be accelerated anyway in the plasterboard line. GB-A-2053178 combines wetting and grinding in one step also in the context of plasterboard production. Scaling is here avoided by shear forces but the DH issue remains unsolved.